Using a combined stable-isotope and fatty-acid approach, we examined carbon-transfer routes from the
cyanobacterium Microcystis to zooplankton in eutrophic Lake Taihu, China. Microcystis is generally considered
poor food for zooplankton, and we hypothesized that most Microcystis carbon flows to zooplankton via dissolved
organic matter (DOM)–bacteria and detritus–bacteria pathways rather than via direct grazing. ... read more The hypothesis
was tested by analyzing 13C isotopes at natural abundance in field samples and in tracer experiments with 13Cenriched
Microcystis. 13C-enriched Microcystis was added as live Microcystis, Microcystis detritus, or Microcystis
DOM to lake-water incubations with Bosmina sp. and Daphnia similis as the dominant species. The 13C isotope
signatures of Microcystis, heterotrophic bacteria, and eukaryotic algae in seston were determined from isotope
analyses of specific fatty acids, and the presence and labeling of these fatty acids were also analyzed in
zooplankton consumers. Bosmina and Daphnia consumed carbon via all pathways, but the amount of carbon
transfer from the Microcystis DOM was the highest, followed by the Microcystis detritus. Bosmina consumed
relatively more live Microcystis than Daphnia. The presence and high 13C enrichment of bacteria-specific fatty
acids in the zooplankton consumers showed that heterotrophic bacteria were an important link between
Microcystis and zooplankton. Microbial pathways dominate the energy flow from cyanobacteria to zooplankton
in eutrophic lakes with heavy cyanobacteria blooms, such as Lake Taihu show less

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